Elemental analysis

Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals, chemical compounds) is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualitative (determining what elements are present), and it can be quantitative (determining how much of each are present). Elemental analysis falls within the ambit of analytical chemistry, the set of instruments involved in deciphering the chemical nature of our world.

For organic chemists, elemental analysis or "EA" almost always refers to CHNX analysis — the determination of the percentage weights of carbon, hydrogen, nitrogen, and heteroatoms (X) (halogens, sulfur) of a sample. This information is important to help determine the structure of an unknown compound, as well as to help ascertain the structure and purity of a synthesized compound.

Methods

The most common form of elemental analysis, CHN analysis, is accomplished by combustion analysis. In this technique, a sample is burned in an excess of oxygen, and various traps collect the combustion products — carbon dioxide, water, and nitric oxide. The weights of these combustion products can be used to calculate the composition of the unknown sample.

Quantitative

Other quantitative methods include:

• Gravimetry, where the sample is dissolved and then the element of interest is precipitated and its mass measured or the element of interest is volatilized and the mass loss is measured.
• Optical atomic spectroscopy, such as flame atomic absorption, graphite furnace atomic absorption, and inductively coupled plasma atomic emission, which probe the outer electronic structure of atoms.

Qualitative

To qualitatively determine which elements exist in a sample, the methods are:

• Mass spectrometric atomic spectroscopy, such as inductively coupled mass spectrometry, which probes the mass of atoms.
• Other spectroscopy which probes the inner electronic structure of atoms such as X-ray fluorescence, Particle-Induced X-ray Emission, x-ray photoelectron spectroscopy, and Auger electron spectroscopy.
• Chemical methods
  • Sodium fusion test
  • Schöniger oxidation

Analysis of results

The analysis of results is performed by determining the ratio of elements from within the sample, and working out a chemical formula that fits with those results. This process is useful as it helps determine if a sample sent is a desired compound and confirms the purity of a compound. The accepted deviation of elemental analysis results from the calculated is 0.4%. The method for working out the ratio of elements from the results is shown below:

1. Take the percentage of each element found and divide by the element's mass. Do this for all the elements for which you have results
2. Find the smallest value from step 1 and divide every value obtained in step 1 by this smallest value
3. Multiply the results in step 2 by a factor to obtain reasonable values for either carbon or nitrogen and then compare to what was expected from a pure sample of the compound that was thought to be submitted

This process is tedious to perform by hand, and automated tools have been released to simplify with this process. Each of the tools is different in its working. CHN+^[1] works under Windows and was designed primarily for discovering solvents occluded in in a compound. The Solvent Correction CHN Calculator^[2] works in a similar manner, but requires an internet connection. The Chemical Composition Calculator^[3] works without an internet connection, calculates elemental analysis on the fly in a user's web browser, but does not iterate to discover occluded solvents, among other items.

References

1. ^ CHN+ http://www.ccp14.ac.uk/ccp/web-mirrors/lcells/chn/downloads.htm
2. ^ Solvent Correction CHN Calculator http://www.che.hw.ac.uk/research/services/solvent.html
3. ^ Chemical Composition Calculator http://www.gazard.com/stephen/chemistry/

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